The invention relates generally to the field of induction motor controllers and particularly to systems for sensing flux indirectly and to control loops for induction motors which attempt to maintain constant flux.
The induction motor is essentially a transformer with its secondary short-circuited and free to move. The fixed primary windings, usually called stator windings or phase windings, are distributed about the axis of a rotor. Typically, the stator windings are interconnected so as to form multiple current paths, and alternating current of different phases of the same frequency is applied to the corresponding windings. The most widely used polyphase induction motor system utilizes three phase excitation.
The secondary conductors on the rotor of an induction motor form a closed circuit. To induce current in the rotor there must be relative motion between the stator flux vector and the rotor itself so that the rotor encounters changing flux. To meet this requirement, the excitation applied to the stator is controlled to establish a rotating flux vector which must in effect spin faster than the rotor. The difference between the stator excitation frequency, .omega..sub.s, in radians per second and the mechanical frequency, .omega..sub.r, representing the shaft speed times the number of pole pairs, is called slip or slip frequency .omega. and is the chief characteristic of all induction motors.
In the "squirrel cage" induction motor, the rotor conductors are spaced parallel bars which are arranged in a cylindrical cage. The bars are connected at the ends of the cage by conductive rings which are affixed to a coaxial shaft journalled for rotation within the stator. The manufacturing cost and maintenance are low for this configuration due to the absence of intricate windings and commutator assemblies for the rotor.
Historically, however, induction motors have been relegated primarily to fixed speed applications. With advances in semiconductor switching technology, it has now become feasible to provide reliable low cost alternating current sources of variable frequency. This ability to control stator current and excitation frequency simultaneously with high accuracy provides an opportunuty for enhanced control of the induction motor at varying speeds and loads. Because of the complicated interrelationship of torque, flux and slip frequency, however, it is difficult to control the torque output of an induction motor. As a result, many otherwise suitable variable speed applications, such as traction vehicles, elevators and servomotors have been left to the more easily controlled varieties of motors, particularly direct current motors. These other types of motors are far more expensive to manufacture and maintain since they require rotor windings and commutators, unlike the simple squirrel cage motor.
It is well known that flux regulation is one of the keys to optimum control of induction motors. Since maximum available torque is proportional to the square of the air gap flux level, it is desirable to operate the induction motor at peak flux in most applications. However, flux is a somewhat elusive function of current level, slip frequency, gap temperature and other parameters. Quasi "open loop" setting of the flux operating level without sensing flux can lead to expensive overdesign, i.e., selection of a motor larger and more expensive than actually required.
Flux can be directly sensed, for example by using Hall effect devices in the air gap between the rotor and the stator. Because this requires altering the motor itself, designing with Hall devices in the gap is costly. Moreover, Hall effect devices are temperature sensitive, exhibit histeresis effects and have very poor noise margin or sensitivity. Extra flux sensing windings which are sometimes used suffer similar disadvantages.
Indirect flux sensing has been attempted by integrating motor terminal voltages and subtracting out the anticipated voltage associated with the known resistance or "IR" drop due to stator current. This technique however requires unusually stable gain in the integrator and becomes inaccurate at low frequencies because of the inherent errors in any open loop IR drop cancellation technique.